1. Field of the Invention
The present invention relates to a technology to separate and analyze nucleic acids, proteins and the like by means of electrophoresis and, for example, relates to a capillary electrophoresis apparatus.
2. Description of the Related Art
Japanese Patent Application Laid-Open No. 2004-144479 discloses a capillary electrophoresis apparatus. The capillary electrophoresis apparatus uses an electrophoresis method using a capillary made of a silica tube and a polymer coating that coats the silica tube for the purpose of determining a base sequence and base length of DNA. A voltage is applied to both ends of the capillary after a sample containing DNA to be measured being injected into a separation medium such as polyacrylamide in the silica capillary. A DNA compound in the sample migrates inside the capillary to generate DNA bands in the capillary after being separated based on molecular weight or the like. Each DNA band has added fluorochrome and thus, develops colors after being irradiated with laser light. By reading such colors using a fluorometric means, the DNA sequence is determined. Constitution of proteins can also be examined by performing separation/analysis of proteins in the same manner.
A method of light irradiation to a sample in the capillary electrophoresis apparatus disclosed in Japanese Patent Application Laid-Open No. 2004-144479 is as follows. That is, a capillary at one end of a capillary array consisting of a plurality of capillaries arranged on a plane substrate or capillaries at both ends of the capillary array are irradiated with a laser light and the laser light crosses the capillary array by propagating successively from one capillary to the adjacent one. A fluorescence detection method is as follows. That is, an image in a laser irradiation part on the capillary array is formed on a secondary CCD through a condensing lens, a transmission grating, and an image formation lens.
If, in a system in which a capillary at one end of a capillary array consisting of a plurality of capillaries arranged on a plane substrate or capillaries at both ends of the capillary array are irradiated with a laser light and the laser light crosses the capillary array by propagating successively from one capillary to the adjacent one, the focal point of the laser light is adjusted on capillaries at both ends, the laser light can efficiently be coupled to the capillary array. Moreover, in a process in which the laser light propagates through a plurality of capillaries, a reflection loss of the laser light is produced on the outside diameter of the capillary based on a difference of the index of refraction between quartz and air. Therefore, if the laser light is radiated from one side, irradiation light intensity will not be uniform among the plurality of capillaries. To minimize intensity variations in the capillary array, the laser light is generally radiated from both sides of the capillary array. Moreover, since a separation medium is present in the capillary tube, the inside diameter and outside diameter of the capillary and the like are determined in such a way that efficiency with which the laser light propagates from capillary to capillary is maximized by adjusting to the index of refraction of the separation medium.
The above conventional capillary electrophoresis apparatus has, as described below, a problem that gel, which is a separation medium, is consumed wastefully. Assume, for example, that six samples are measured in a capillary electrophoresis apparatus having 16 capillaries. If the capillary is filled with air, the efficiency with which exciting irradiation light propagates from capillary to capillary declines compared with a case in which the capillary tube is filled with a separation medium because the index of refraction in the capillary tube becomes smaller. Thus, a problem of rising running costs exists because the separation medium must be injected into all 16 capillaries even though the number of samples is six.
If capillaries consist of two independently removable capillary arrays, each of which having eight capillaries, and six samples are measured, the above problem of wastefully consuming the separation medium can be solved by means of a system that allows removal of one capillary array when six samples are measured. In this case, however, another problem arises. If both capillary arrays are radiated from one side, signal intensity variations increase among capillaries. If two capillary arrays are irradiated with exciting irradiation light on both sides and one of the capillary arrays is removed, a problem arises that coupling efficiency of exciting irradiation light to the capillary arrays declines because no capillary will be present at a focal point of light.
The present invention has been developed in view of the above situation and an object thereof is to provide a capillary electrophoresis apparatus capable of mounting 2n (n is a positive integer) capillary arrays, wherein coupling efficiency of exciting irradiation light to the capillary arrays does not decline even if any one capillary array is removed.